1. Field of the Invention
The present invention relates in general to a blood pressure monitor system and in particular to such a monitor system which continuously determines blood pressure values of a living subject, according to a relationship between blood pressure and pulse wave magnitude, based on detected magnitudes of the pulse wave.
2. Related Art Statement
There is known a blood pressure monitor apparatus of the type which (a) presses a body portion (e.g., upper arm) of a patient by inflating a cuff wound around the upper arm, (b) detects an oscillatory pressure wave transmitted from the upper arm to the cuff in synchronism with heartbeat of the patient, or a start and an end of arterial sounds (i.e., Korotkoff sounds) produced from a brachial artery in the upper arm of the patient, while the air pressure in the inflated cuff is slowly lowered, (c) measures a systolic and a diastolic blood pressure of the subject based on the variation of the oscillatory pressure wave or based on the start and end of the Korotkoff sounds, (d) detects a pressure pulse wave produced from an artery (e.g., radial artery) of the patient by using a semiconductor pressure sensing means, (e) determines a relationship between blood pressure and pulse wave magnitude (hereinafter, abbreviated to the "BP-PW relationship"), based on the systolic and diastolic blood pressure values measured by using the cuff and two pulse wave magnitudes detected by the pressure sensing means, (f) updates the BP-PW relationship at predetermined intervals of time, and (g) continuously determines or estimates, according to the currently effective BP-PW relationship, blood pressure values of the patient based on detected magnitudes of the pressure pulse wave. Thus, this apparatus is capable of monitoring the blood pressure of the patient for a long time, without so frequently pressing the upper arm of the patient by inflating the cuff. This apparatus is disclosed in U.S. Pat. No. 4,928,701 assigned to the Assignee of the present application.
In the above-identified monitor apparatus, the BP-PW relationship is determined by solving the simultaneous equations of first degree with two unknowns in which a measured systolic blood pressure is associated with a detected upper-peak (maximum) magnitude of a pulse of the pressure pulse wave and a measured diastolic blood pressure is associated with a detected lower-peak (minimum) magnitude of the same pulse or another pulse. The BP-PW relationship is expressed by the following linear function: BP=a.multidot.M+b, in which BP represents blood pressure, M represents pulse wave magnitude, and a and b are constants. Therefore, the accuracy of determination of blood pressure values by this apparatus depends on the accuracy of the BP-PW relationship. It is however known that, generally, diastolic blood pressure values of a subject measured by using an inflatable cuff suffer from considerably large variance. Since the apparatus determines and updates the BP-PW relationship based on diastolic blood pressure values that may be largely variable, the apparatus may not accurately determine or estimate blood pressure values of the subject.
In addition, in the case where the cuff is set upstream of the pulse wave sensing means on the same and one limb of the patient, the pulse wave sensor cannot detect the pressure pulse wave from the radial artery and therefore the apparatus cannot monitor the blood pressure of the patient, during the periods of measurement of systolic and diastolic blood pressure values using the cuff, because the blood flow from the brachial artery to the radial artery is blocked by the inflated cuff.